Shape Fabrics and Superimposed Simple Shear Strain in a Precambrian Shear Belt, W Greenland

J. geol. Soc. London, Vol. 136, 1979, pp. 471-488, 12 figs. Printed in Northern Ireland.

Shape fabrics and superimposed simple shear strain in a Precambrian shear belt, W Greenland

John Grocott

SUMMARY: Shapefabric type in thenorthern part of the IkertBq shearbelt and the adjacentarea has been recordedboth qualitatively andquantitatively, and its variationis described. Shape fabric variation is shown to be closely related to structural changes, and both are reconciled with a deformation model involving superimposed strain. In theshear belt itselffinite strainshape fabrics are shown tobe aconsequence of superimposition of 2 differently orientated simple shear strains. The earlier simple shear strain initiated the shearbelt and was characterized by horizontal ENE-WSW movements in avertical shear plane. The second deformation was produced by overthrusting in a SSE direction. In effect the XZ plane of the later deformation was superimposed on YZ sections through shape fabrics produced by transcurrent movements. The overthrusting deformation was superimposed on earlier structures with gradually increasing intensity, allowing the resultant deformation path to be recorded in detail. A steep strain gradient marks the northern boundaryof the shear belt, butlow deformation of shear belt age canstill be recognized further N. Shape fabric variation N of this limit of intense strain is described.Structural and shape fabric variationin this area is aconsequence of superimposition.. of low magnitudes- of overthrusting simple shearstrain on pre-shearbelt structures and fabrics.

The IkertBq shear belt is a major zone of intense Nag. 2 respectively, separated by a swarm of basic ductilestrain in the Precambrianbasement gneiss dykes, called Kangfimiut dykes. Inthe W, strong complex of western Greenland. Compelling evidence Nag. 1 deformation is recognized across much of the accumulated during earlier work in the shear belt has shear belt, but not in an area of low strain centred on led to the adoption of a simple shear model as a basis Kingaq, orat the extreme northern margin of the for its structural interpretation (Escher & Watterson shear belt. Nag. 2 strain is absent immediately S of the 1974; Escher et al. 1975). Despite a weight of sup- Kingaq augen in the Itivdleq district, but increases porting evidence, shape fabrics in the shear belt are gradually northwards culminating in a 7 km wide zone generally not of the plane strain type demanded by of intense deformation along the northern boundary this model. This paper describes shape fabric variation (Fig. 1). Apart from the boundary of the shear belt SE in theshear belt, together with the accompanying of SandreStr~mfjord, where coincident facies and structural changes. An important aim is to describe deformation boundaries mark the southern limit of a how shape fabrics and structures evolve in large scale zone of Nag. 2 strain, the distributionand relative shear belts as a result of more than one deformation. intensity of the component deformations in the rest of Trending ENE-WSW, the IkertBq shear belt has an the shear belt are unknown. exposed length of 150 km to the inland ice, and is of Ramsay & Graham (1970) showed that fabric ele- variable width (Fig. 1). The dominant lithology is ments formed in simple shear zones reflect displace- granodioritichornblende and biotite-amphibolite ments across such zones. Accordingly, where facies gneiss. The shear belt is flanked Nand S by Kangfimiut dykes are notdeformed, vertical ENE- gneisses which are dominantly granodioritic, with min- WSW, or E-W planar elements of shape fabrics (folia- eral assemblages of the hornblendegranulite facies. tion) coupled with sub-horizontallinear elements These rocks are largely unaffected by deformation (stretching directions) show that movements were dex- responsible for forming the shear belt. Whilst at the tral and sub-horizontal within an approximately E-W southern boundary of the shear belt, facies and defor- vertical shear plane(Bak et al. 1975). Where the mation boundaries coincide, at the coastal part of the dykes are intensely deformed, Nag. 2 strain is charac- northern boundary the steep strain gradient is 5 km N terized by overthrustingmovements within a shear of the facies boundary (Fig. 1).In this 5 km wide zone plane dipping 20"-50"NNW (Escher et al. 1975; the gneisses were reworked in granulite facies, whilst Grocott 1977). elsewhere in theshear beltthey were retrogressed As will be demonstrated, many conclusions are built during reworking (Grocott, in press). on the belief that in rocksnot containing large The IkertBq shear belt occurs within the Nagssugto- amounts of mica, shape fabrics can be used to indicate qidian of western Greenland. The 2 principal phases the orientation and magnitude of the finite strain, and of deformation recognized in it are called Nag. 1 and that they also reflect the ellipsoid type (oblate, prolate

0016-7649/79/0700-0471$02.00 @ TheGeologicalSociety

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or whatever) describing the finite strain (Flinn 1965; Shapefabrics in the gneisses are of planestrain Ramsay & Graham 1970). type, belonging to the LS class of the LS fabric system It is useful to distinguish 3 types of macroscopic (Flinn 1965). They give the orientation of the Nag. 1 fabric. Shape fabrics usually consist of quasi-elliptical strain ellipsoid in this area (Fig. 3). aggregates of many mineral grains. Quartz is an excep- In central Sagdlerssuaq both dykes and gneisses are tion at high metamorphic grade, and consists of len- folded. The folds are open with 'S' asymmetry and soid grains made up of relatively few individual grains. verge to the S. They are believed to be Nag. 2 in age. Such differences in texture reflect different deforma- Axes plunge gently W, parallel to theearlier stretching tion mechanisms (White1976). Nevertheless, in the fabric in steeply dipping or vertical axial planes gneisses which arethe subject of this paper,shape (Fig. 24. Nag. 1 shape fabrics arereorientated in fabrics formed by quartz do not differ in orientation these folds, but arenot modified from LS type. In from those formed by other minerals in any one rock. profile section they exhibit a shallowly dipping short Mineral fabrics, unlike shape fabrics, are defined by limb, and a steep long limb with bandingparallel the crystallographic orientation of individual mineral shape fabrics (Fig. 4). This style is important when grains such as biotite and opaques (Watterson 1968). effects of superimposed strain are considered. Where the last deformation is strong, shape and mineral fabrics are parallel. In contrast,mineral fabrics Area 2 andshape fabrics are not parallel where the last This area includes the 7 km wide zone of intense deformation is weak, showing that mineral fabrics do Nag. 2 deformation at the northern boundary of the not reflect the finite strain. A thud fabric often de- shear belt (Fig. 1). From the southern coast of Sarfan- veloped in gneisses is simply a lithological variation on guaqlandstrain increases northwards into thiszone. a scale greater than that induced by the formation of The most strongly deformed rocks occur on NW Sar- shape fabrics. This fabric is called banding. fanguaqland and then strain begins to decrease north- Throughout this paperprolate shape fabrics are wards. This decrease becomes marked at a deforma- termed L tectonites, fabrics in the constrictional field tion boundary which can be mapped on Umanarssugs- L > S tectonites,plane strain fabrics LS tectonites, suaq andsouthern Manitsorssuaq. Thisboundary fabrics in the flattening field S>L tectonites,and marks the northern limit of high Nag. 2 strain (Fig. l), oblate fabrics S tectonites(after Flinn 1965). The and further N shape fabrics become variably orien- parameter k (Flinn 1965) forthese fabrics takes values tated. It is taken as the northern boundary of area 2 of m, m> k > 1, 1, 1> k >O and 0 respectively. and of the shear belt. Where Nag. 2 deformation is The work reported here relates to the coastal part of most intense the ,foliation strikes 065" and dips 55" the shear belt N of Kingaq, and distinguishes 3 struc- NNW with adown-dip stretching direction. These tural areas. From S to N, area 1 consists of western fabricspermit the orientation of the Nag. 2 shear and centralSagdlerssuaq, area 2 of western Sarfanguaq- plane to be estimated (Fig. 3). The effect of superim- land,southern Umanarssugssuaq and part of south- position of different amounts of Nag. 2 strain on ear- ern Manitsorssuaq, and area 3 of western Manitsors- lier structures and fabrics is described in detail in suaq andnorthern Umanarssugssuaq (Fig. 1). Only sections 2 and 3. areas 2 and 3 are discussed in detail. Area3 Area1 This is N of the steep gradient in Nagssugtoqidian On western Sagdlerssuaq, vertical foliation in strain which crosses Umanarssugssuaq andsouthern strongly deformed gneisses trends 075" associated with Manitsorssuaq and which is taken as thenorthern a shallow westerly-plunging stretchingdirection boundary of the shear belt. Low or moderate Nag. 2 (Figs. 2a,b). The foliation is axial planar to isoclinal strain can nevertheless be recognized at most pre-dyke folds of the banding which plunge parallel to localities. Measurement of shape fabrics across Manit- the stretchinglineation (Fig. 2c). Kanghmiut dykes sorssuaq shows that Nag. 2 strain gradually increases show persistent slight discordances to banding, southwardsas this deformationboundary is ap- trending 5-10' more north-easterly. Many dykes re- proached. This increase progressively modifies earlier tain primary intrusive structures including discordant shape fabrics and structures which are shown to be apopheses, and some have primary igneous textures. pre-Nagssugtoqidian. Althoughappearing undeformed externally, most dykes have shape fabrics whose orientation is almost parallel tothat in the surrounding gneisses. These Shape fabric and structural fabrics are believed to result from deformation during variation in area 2 cooling from igneous temperatures, and after strong deformation of the countryrocks during Nag. 1 Variation in shape fabric type in areas 2 and 3 is (Bridgwater et al. 1973; D. F. Nash, pers. comm.). shown on Fig. 5. 6 structural sub-areas are recognized

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N. 29 points N. 87 points

FIG. 2. Structural data, Sagdlerssuaq. a, Poles to foliation and banding; b, Stretching lineation; c, Pre-dyke minor folds; d, Poles to banding in areas of post-dyke folding. Contours at l%, 3% and 5% per 1% area. in area 2, and are described approaching the shear belt 6g). Axesplunge parallel tothe stretchingfabric boundary from S to N across area 2. Structural maps (Fig. 6g). which complement Fig. 5 can be obtained either from Planarzones of intenseductile deformation theauthor, or (uponpayment) fromthe Geological between 1 and 50 m wide cut this sub-area. Post-dyke Society Library, or the British Library Lending Divi- folds are tightened in these flag-like zones, and paral- sion, Boston Spa, Yorkshire, UK (as Supplementary lelism of isoclinal fold axes gives an intensely rodded Publication No. SUP 18029, 2 pages). appearance to the gneiss. Despite this the stretching direction is often weak although the foliation is obvi- ous. Stretching fabric orientation is similar to that in Sub-area 2A the surrounding gneisses. Kanglmiut dykes arede- This sub-area is characterized by L and L > S tec- formed into these zones and become concordant, de- tonite fabrics both in gneisses and in Kangdmiut veloping LS or S > L tectonitefabrics throughout. dykes. Stretching lineations plunge gently W, slightly The stretchinglineation within the dykes does not more westerly thanthose in area 1 (Fig. 6a). Fabric change orientation as the dykes are deformed. Dis- elements in both dykes and country rocks are parallel, placement of dykes across thesezones has a small though most dykes are discordant to varying degrees horizontalcomponent with adextral sense. Actual to banding in the gneisses. displacements are unknown. In this sub-areathere are many post-dyke folds. Axes and axial planes of all folds are parallel. Axial Sub-area 2B planes trend 065" and dipapproximately 50"NNW, parallel to the foliation in sub-areas further N where Gneisses in this sub-area appear superficially similar Nag. 2 deformation is stronger (compare Figs. 6f and to those in western Sagdlerssuaq (area 1) in that they

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contain LS shape fabrics, the linear element of which often plunges gently W. Furthermore these rocks are unaffected by post-dyke folds. Foliation and banding are parallel except in the hinges of intrafolial pre-dyke folds which plungeparallel to the stretchingfabric. The foliation strikes more NE than in area 1, dipping NNW (Fig. 6f),and significantly the stretching lineation plunges more steeply to the NW in the northern part of the sub-area (Fig. 66). Kanghiut dykes trend up to 5" more north-easterly thanthe foliation, and contain LS fabricsvirtually parallel to those in the country rocks. They are more thoroughlyrecrystallized than dykes in area 1, and primaryigneous textures and intrusive features are absent. Planarzones of intenseductile deformation up to 30 m wide cut these rocks, resembling those described in sub-area 2A. However they do not contain post-dyke folds. They are parallel or nearly parallel to foliation in the surrounding gneisses, and the stretching lineation also has the same orientation as outside the zone. Shape fabric type becomes S > L as deformation increases. Kangsmiut dykes are deformed into these zones, and show a small dextral horizontaloffset. EIG. 3. Orientation of superimposed Nagssugtoqi- dian strains inferred from shape fabrics where each strain is intense.The simple shearmodel is as- Sub-- 2C sumed for each strain and in both diagrams the undeformed condition is represented by a cube. Each cube is then transformed to a parallelepiped Stretching lineations vary in orientation in this sub- by displacements within the outlined shear planes. area betweenshallow westerly plunges andsteeper Appropriate finite strain axes are inscribed. Notice plunges to the NW (Fig. 6c). Judging from the degree that X2 of Nag. 2 strain is sub-parallel to YZ of of dyke deformation, stretching lineations orientated Nag. 1 strain. towards the NW occur where Nag. 2 strain is more intense. In the SW of the sub-area shape fabric type is S > L, though this rapidly becomes LS further N. The orientation of the foliation is similar to that elsewhere in area 2 (Fig. 6f).Fabrics within Kanghiut dykes are S. N. parallel to those in their country rocks, and the dykes are completelyrecrystallized and contain LS fabrics throughout. No discrete mnes of especially intense strain occur and the gneisses, exceptthose described below in sub-area 2E, are uniformly flag-like, suggesting that the last deformation is high. Banding and foliation are parallel, except in the hinges of intrafolial pre-dyke folds which plunge parallel to the stretching fabric.

\ Snb-meCl2D The facies boundary in this part of the shear belt doesnot coincide with deformationa boundary. Stronglydeformed gneisses containsimilarly orientated LS tectoniteshape fabrics in sub-areas 2C and 2D on each side of the facies boundary (Fig. 5). These sub-areas are distinguishedstructurally on a FIG. 4. Profile of a majorearly Nag. 2 fold in basis of overall stretching fabric orientation, and on central Sagdlerssuaq. the deformation intensity of Kanghiut dykes.

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In sub-area D the stretching lineation is more con- approaching this enclave there is no change in shape stantly orientatedthan further S in sub-area C, al- fabric type or orientation. though a 'tail' of data towards shallower, more wes- A swarm of hypersthene-bearing basic dykes occurs terly plunges still exists (compare Figs. 6c and d). in this sub-area.They may beconcordant with the Again minor pre-dyke folds plunge parallel tothe foliation, but often show marked discordances up to local stretching lineation, with axial planes parallel to 15". These dykes contain strong LS tectonite fabrics, the foliation.Post-dyke folds only occur in theen- the linear element plunging down the dip of the folia- claves described later. Exceptingthese enclaves, the tion, which is parallel to the margins. Occasionally the deformation is remarkably homogeneous and strong. dykes are folded, but they are usually parallel-sided; Kangiimiut dykes are more strongly deformed in they are believed to be Kangiimiut dykes less de- sub-area D, and containmetamorphic hypersthene. formedthan in the northernpart of sub-area 2D. Width never exceeds 2 m, and the dykes are concor- N from sub-area2F Kangdmiut dykes become less com- dant and contain strong LS tectonite fabrics. Towards mon and are not recognized on northern Manitsors- the N of the sub-area, dykes are disrupted and form suaq, or around Holsteinsborg. basic schlieren. Interpretation of shape fabric Sub-area 2E and structural variation In sub-areas 2B, C and D there are lensoid enclaves in area 2 in which shape fabrics are weakerthan in thesur- rounding gneisses. These enclaves contain many post- To interpret tectonite fabrics and structures in terms dyke minor folds. Almost all differ in structural detail of superimposedhomogeneous deformation, the as a consequence of different strain histories, but all orientations of the component strains must be known. have the above two points in common. In the IkertBq shear belt N of Kingaq, if the orienta- The largest enclave occurs on NW Tinorqassarssuaq tion of shape fabrics formed in areas of intense Nag. l in the core of a westerly plunging synform. Banding and Nag. 2 deformation respectively are considered, it and Kangdmiut dykes are grossly discordant. Shape is evident that the strike of the foliation produced by fabrics are LS in type, the stretching lineation plunges each is similar. Where these strains overlap, the orien- NW,and thefoliation is parallel to that elsewhere on tation of the strain axes relating to each deformation is Sarfanguaqland. Post-dyke fold axial planes are paral- therefore such that the XZ plane of Nag. 2 deforma- lel to thefoliation, and fold axis oriention is variable. tion is similar in orientation to the YZ plane of shape Other enclaves on Tinorqassarssuaq have structures fabrics formedduring Nag. 1 strain on which it is transitional between those just described, and those in superimposed. Accepting a simple shear model for sub-area 2A. Post-dyke fold axes become more con- thesedeformations, their relativeorientations are stantly directed towards the W, and the shape fabric shown in Fig. 3. The Nag. 1 shear plane N of Kingaq is become L > S in type, with westerly plunging stretch- vertical and trends 075", whilst the Nag. 2 shear plane ing lineations. dips 50-335". Further N post-dyke strain within and outside these enclaves is stronger, and as this increases northwards The significance of early Nag. 2 folding post-dyke folds within the enclaves are progressively In western Sagdlerssuaq (area 1) intensely deformed rotatedto the NNW and attaina steeper plunge flag-like gneisses containing minor folds with axes (Fig. 6g). A stretching lineation plunges parallel to the parallel to sub-horizontal stretching fabrics, and fold axes, and has a similar orientation to that in the slightly deformed Kang2miut dykes, are interpreted as surrounding gneisses. Together these elements give zones of intense Nag. 1 simple shear strain. Apart the gneisses arodded appearance. The foliation is from an 075" as opposed to 090" strike, the geology is axial planar to the folds, and has the usual orientation essentially similar to thatof the Itivdleq district further forarea 2, but never seems particularly prominent. S (D. F. Nash, pers. comm.). However, a generalization about fabric type in these Parallelism of pre-dyke fold axes and the stretching enclaves N of the facies boundary cannot be made. fabric is believed to be a consequence of rotation of fold axes during this strain(Escher & Watterson 1974). Further E where Nag. 2 folds occur, they are Sub-area 2F open structures in which there is little modification of In these granulite facies rocks, structures are similar earlier shape fabrics, but these folds also plunge paral- to those in sub-area 2D. The stretching fabric plunges lel to the stretching fabric. Since Nag. 2 strain judged even more uniformly NNW within a constantly orien- in terms of shape fabric modification is weak in central tated foliation (Fig. 6e).Shape fabric type is uniformly Sagdlerssuaq, the orientation of post-dyke fold axes LS except in a small enclave in the W where shape parallel to theearlier stretching fabricis believed to be fabrics are absent (Fig. 5). Asstrain decreases primary.

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In L>S tectonites

In f lag-like gneiss

In Kangimiut dykes

In L tectonites

N. 71 Doints

In LS tectonites

In flag- like gneiss

In Kangamiut dykes

N. 69 points 72 points

FIG. 6. A-E, Stretching lineations in sub-areas 2A,2B, 2C, 2D and 2E respectively. f, Poles to foliation. g, Post-dyke fold axes (4, 5, 6) and poles to axial planes (7, 8, 9) from sub-areas 2A and 2E. 4 and 7 are from the S (sub-area 2A), 5 and 8 from the centre and 6 and 9 from the northern part of western Sarfanguaqland. Contours at l%,3% and 5% per 1% area.

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Prior to folding, foliation and banding were proba- N. 61 points bly vertical, trending 075" in central as well as western parts of area 1. Planar structures with this orientation are not in the shortening field of an ellipsoid resulting from a small amount of Nag. 2 simple shear in a shear plane dipping 50"NNW. (Their dominantly 'S' asymmetry shows they cannot have formed due to irrota- tionalstrain in which the plane of flattening dips 55" NNW). The position of the shortening field of the weak Nag. 2 strain on Sagdlerssuaq may differ fromthat further N. Such variation may occur in simple shear zones due to boundary effects asthey propagate (Coward 1976). Thus in areas of low Nag. 2 strain the shear plane may be steeper, so that vertical planar structures are in the shortening field. The 'S' asymmetry of post-dyke folds on Sagdlerssuaq is consistent with this suggestion. Alternativelythese folds may duetobe heterogeneities in earlyNag. 2 strain rather than buck- N. 100 points ling. Such a process is difficult to envisage since many short limbs of these folds dip less than 50"NNW and some even dip S, implying that some folded planes have rotated through the shear plane. Orientation of these folds parallel tothe earlier stretching lineation is easier to account for. As the Y axis of Nag. 2 strain is contained within the planar structures prior to folding, and is approximately parallel to the stretching fabric within these planes, Nag. 2 folds form by rotation of their limbs about the earlier stretchingfabric irrespective of the mechanism in- volved in their formation. In many deformation belts late fold axes are parallel to an earlierstretching fabric. Superimposition of weak later strains in orientations which are not ran- dom with respect of earlier fabrics may be generally responsible. Early Nag. 2 folding on central andeastern N. 70 points Sagdlerssuaq occurs on a large scale, and Nag. 1 fabrics are reorientated over large areas. During superimposition of homogeneousstrain on thesestructures, variation in shape fabricorientation would result in profoundlydifferent deformation paths in different areas. Large scale folding of Nag. 2 age also occurs in area 3. It is reasonable to suggest that during early Nag. 2 strain such folds formed throughout the north- em part of the shear belt. Shape fabrics and structures in area 2 will now be interpreted with this probability in mind.

Structures and shape fabrics in area 2 Sub-areas 2A and 2E Fig. 7 illustrates the effects of superimposition of an overthrusting simple shear strain on Nag. 1 shape fabrics of varying orientation (such as might be intro- duced by early Nag. 2 folding). Theseresults were Fig. 6 (continued). obtained using a shear box containing cards on which

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25*N NW. 0" 25"SSE. A

YZ SECTION, NAG.l STRAIN (5.0:1.0:0.2) ri/ V / / k3 /

FIG. 7. Resultant fabricsproduced by superimposition of increasing Nag. 2 strain (left column)and variably orientated banding-parallel Nag. 1 shape fabrics (top row). All the resultant ellipses shown remain YZ sections of the finite strain. Resultant ellipsoid type is indicated by a value for k.

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an ellipse representing pre-existing fabrics was in- forms the core of a Nag. 2 synform (Fig. 5), and it scribed and then deformed by shearing the cards. seems possible thatthe area of low Nag. 1 strain This approach shows that after moderate amounts ‘nucleated’ the fold. of superimposed strain, L tectonites will form in dykes Occurrence of large numbers of post-dyke minor and gneisses where the reorientated foliation dipped folds in sub-areas 2A and 2E may reflect the fact that gently NNW (Fig. 7). This suggests that sub-area 2A the banding and foliation rotated into the shortening formed a short shallow-dipping limb of an early Nag. 2 field of the Nag. 2 strain ellipsoid during early Nag. 2 fold subsequently modified by moderate Nag. 2 folding. homogeneous simple shear strain. Deformation beyond the L tectonite stage will result 2B in the deformation path following a course from L to Sub-area L > S through LS and S > L (all with shallow Fig. 7 reveals an interestingpoint concerning the westerly-plunging stretching fabrics) to S. At very geometry of superimposed strain. When the foliation high strains S > L tending to LS fabrics are predicted formed by the earlier strain dips in a direction close to with down-dip stretching fabrics. More strongly de- the shear plane of the superimposed strain, very little formed planar zones in sub-area 2A areinterpreted as modification in shape fabric type or strain ratio occurs zones of high Nag. 2 strain in which this deformation until superimposition shear strains exceed 2. There is path has reached S > L with shallow westerly plung- no equivalent position when the superimposed strain is ing stretching fabrics. Despite the fact that these are irrotational, for in such cases substantial modification zones of strong superimposed overthrusting deforma- always occurs when strains of this magnitude are tion, the stretchingfabric retains asub-horizontal reached. orientation. Effects of this type must always be consi- Exceptfor NNW dips, gneisses in sub-area 2B dered in relatingstretching lineation orientation to contain LS tectonite fabrics and are similar to gneisses movement directions of a particular strain. on western Sagdlerssuaq. However, Kangsmiut dykes All the numerous Nag. 2 minor folds in sub-area 2A show much more extensive deformation and recrystal- plunge parallel to the stretching lineation since, as in lization, and become concordant within planar zones area 1, the Y axis of Nag. 2 strain lies in the banding of intense post-dyke deformation. At higher strains approximately parallel to the earlier stretching fabric. within these zones the deformation path moves from Thus, irrespective of their mechanism of formation, LS to S > L retaininga shallow westerly plunging these folds formed by rotation of their limbs about the stretchinglineation. Fig. 7 shows that slow earlierstretching fabric, so their axes will plunge movement along adeformation path from LS to parallel to it S > L is consistent with earlier fabrics dipping NNW At this point it is appropriate to interpret the en- atabout 50” prior to superimposition of Nag. 2 claves of relatively weak shape fabrics in area 2 (sub- homogeneousstrain. Such an orientationof earlier area 2E). Structurally they are similar to sub-area 2A, fabrics would occur in the long northerly dipping limb andare interpreted in the same way as shallow of a large-scale early Nag. 2 fold (cf. Fig. 4). northward-dipping limbs of large scale folds formed during early Nag. 2 strain, and later modified to vary- Sub-areas 2C and 2D ing extents by Nag. 2 homogeneous strain. Northwards across area2 these enclaves are progressively more Fig. 7 implies that at sufficientlyhigh values of deformed during Nag. 2, and fold axes concomitantly superimposedstrain all shape fabrics in area 2 will rotate towards the NNW. Apart from rotation of fold pass through an S tectonitestage, after which a axes, enclaves in sub-areas 2C and D resemble planar stretching fabric orientated down the dip of the folia- zones of high strain which have a rodded appearance tion will develop. As Nag. 2 strainincreases north- in sub-area2A, andthey are similarly believed to wards across sub-areas 2C and 2D a broad zone with S reflect effects of stronger Nag. 2 strain and to have or S > L shape fabrics would be expected. Similarly, a followed a similar deformation path. northwards change from shallow westerly to moder- The enclave on NW Tinorqassarssuaq differs in that ate NNW (down-dip) plunging stretching lineations on post-dyke minor folds are variably orientated, and the either side of a zone of S tectonites would be consis- weak shape fabrics are LS in type. Both these observa- tent with the proposed model. tions are consistent with low Nag. 1 strain. In this case Thisdoes not occur. Instead the stretchingfabric variablebanding orientation (‘unstraightened’ by rotates gradually to the down-diporientation across Nag. 1 deformation) results in the variable orientation sub-area 2C, and S> L tectonites occur only locally. of post-dyke fold axes. In addition, the absence of LS fabrics predominate in both these sub-areas, but strong shape fabrics, acharacteristic of parts of the very largestrains are required to produce them if pre-Nagssugtoqidian on eachside of the shearbelt, strain is superimposed in the strictly coaxial way would allow those formed as a consequence of Nag. 2 suggested in Fig. 7. deformation to reflect this strain directly. This enclave Departures from the model are believed to result

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from inexact coincidence of the Y axis of Nag. 2 strain with the X axis of the Nag. 1 shape fabrics. As a NNW. result,the long axis of the finite strain ellipsoid gradually rotates towards the X axis of the Nag. 2 strain ellipsoid as strain progresses. These arguments alsoapply to reorientation of fold axes. At low superimposedstrains the predictions of the model appear to hold, but at higher strains increasingly seri- ous discrepanciesarise in the type, orientationand magnitude of the resultant shape fabric it predicts. In sub-area 2D uniformly NWplunging stretching lineations over much of the area and strong deformation of Kanggmiut dykes testify tothe intensity of Nag. 2strain. Fig. 6d shows a ‘tail’ of data towards shallow westerly plunges reflecting areas of lower a. Pre-Nag 2 Nag.2 deformation within sub-area 2D, and the pres- ence of Nag. 1 strain. Structures over most of these sub-areas are consistent with superimposition of Nag. 2 strain on originally steeply dipping foliation and banding, possibly on the steep northward dipping limbs of early Nag. 2 folds. However, for the reasons mentioned above, it is un- likely that S tectonites formed, the deformation path being LS through S > L to approximately LS.

Sub-area 2F Although Kanghmiut dykes in this sub-area are less 2C I 2E 1 28 I 2A deformed than in sub-area 2D, the orientation of the stretching lineation becomes more constant (Fig. 6e). This suggests that pre-Nag. 2 strain was weak rather L than Nag. 2strain was especially strong. In fact no change in the type ororientation of shape fabrics occurs approaching the enclave of low deformation in western Umanarssugssuaq, and absence of shape fabrics in this enclave further suggests that both Nag. 1 and pre-Nagssugtoqidianstrains are low. Conse- quently the northern boundary of intense Nag. 1 deformation is drawnbetween sub-areas2D and 2F, C. several km S of the steep gradientin Nag. 2 strain which marks the northern boundary of the shear belt (Fig. 1). FIG. 8. Structural evolution of area 2 shown as a series of sections. a, Re-Nag. 2 strain; ellipses summary represent vertical foliation formed during Nag. 1 deformation. A low deformation area is shown, viz. The conclusions of this section are summarized in sub-area 2E on NW Tinorqassarsuaq. b, Reorien- Fig. 8, which depicts the northern part of the shear tationduring early Nag. 2 folding. c, Resultant belt after the Nag. 1 deformation. Later deformation shapefabrics after homogeneousNag. 2 strain. is shown as a two stage event.Firstly the gneisses were Strainin each sub-areadepends on the initial folded by an early heterogeneous phaseof Nag. 2, and fabricorientation and the intensity of Nag. 2 subsequent homogeneous strain resulted in the struc- homogeneous strain. tural history recorded here. large scale domeand basin-like folds. Southwards Shape fabric andstructural across the island, Nag. 2 strain is shown to increase, variation in area 3 and large scale foldstighten. Further S, in sub-area 2D, suchfolds are isoclinal, andthe superimposed Shape fabric type is highly variable across Manitsors- strain so intense that variation in shape fabric is small. suaq, and changes systematically with location within In area 3 qualitative estimates of shape fabric type

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and intensity have been supplemented by strain measurements. The rock type lending itself to measurement is a quartz-microcline gneiss in which lensoid quartz grains defining a shape fabric were measured.

Structure Thestructure of western Manitsorssuaq is domi- P nated by 2 basin-like folds (indicated on Fig. 5) separated by anantifonn (see Fig. 10). The southerly synform is tighter, and can be traced onto the N coast of Umanarssugssuaq. The northern boundary of intense Nag. 2 strain forms the southern boundary of area 3. Mineral fabrics, defined chiefly by biotite and pyr- rhotite have a constant 065" strike across the islands, but their dip steepensnorthwards (Fig. 9a). The linear element of the mineral fabric was only measured at a few localities, and plunges down the dip of the planar element. Planar (S) or S >L shape fabrics occur in the northern and southern limbs of each synform, while L tectonites occur in theeastern hinge zones. Inthe western hinge zone of the northern fold L > S tectonites predominate, but closer to the core of the fold S >L tectonites occur (Fig. 5). , The foliation trace is parallel to the banding trace on horizontal surfaces. In vertical section the foliation lies nearer the plane of the mineral fabric than the banding (Fig. 10). L or L > S shape fabrics occur throughout the hingezone of the southern synfonn, but are absent fromthe core of thenorthern fold (Fig. 5). Here foliation and banding are parallel, andthe mineral fabric weak or absent. The shape fabric is of LS type, and shows a low strain. The stretching lineation plunges down the dipof the foliation in all the fold limbs except the northern fold limb of the northern synform in which it has a shallow plunge to the W. In the westernand eastern hinge zones of themajor folds the stretchinglineation plunges E and W respectively within the banding (Fig. 9b).

Strain measurements Long and short axes of lensoid quartz grains defining shape fabrics in quartz-microcline gneisses were measuredin samples from 12 localities (Figs. 5, 11). To take into account the initial shape factor, the methods suggested by Dunnet (1968) were employed on 3 mutually perpendicular non-principal planes cut through each specimen to determine the strain ellipse in each surface. The type and magnitude of the strain ellipsoid is determined by solving equations derived by Ramsay (1967, p. 199) using a programme in Roberts & Siddans (1971). This programme derives an internal inconsistency factor from which the accuracy of the measured data can be judged. The measuredstrain

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UNW. SSE.

N.W. Manitsorssuaq S. Manitsorssuaq N.W. Sarfanguaqland

FIG. 10. Sectionacross area 3. Ellipseindicates the foliation trace. thin broken lines indicate the trace of the mineral fabric. values confirm the qualitative assessments made in the Interpretation of shape fabric field, and highlight the increase in strain southwards and structural variation across area 3 (Fig. 5). in area 3 The component strains Theshape fabricvariation in area 3 can be ac- counted for by the superimposition of 2 homogeneous strains. The northern limit of Nag. 1 deformation has been recognized in area 2, and it is argued that in area 3 variations in shape fabric are due tosuperimposition of Nag. 2 deformation on pre-Nagssugtoqidian structures and fabrics. Mineral fabrics in area 3 have orientations typical of Nag. 2 deformation, suggesting that Nag. 2 is a component of the finite strain. The planar element of these fabrics steepens progressively northwards (Fig. 9a). This may indicate that the mineral fabric reflects the orientation of the Nag. 2 rotational strain ellipsoid, the planar element of which must steepen northwards as Nag. 2 strain decreases. The orientationof pre-Nagssugtoqidian fabrics upon which this strain is superimposed is more difficult to define. N of Holsteinsborg large scale folds are open structures, and they tighten towards the shear belt as the axial planar Nag. 2 mineralfabric becomes stronger. At the same time they become increasingly overturned towards the S. These observations clearly suggest that their formation is related to Nag. 2 deformation. The folds have aflat-lying envelope (Davidson 1978). In the area N of Holsteinsborg where folds are beginning to develop, weak shape fabrics of approxi- FIG. 11. Strain plane plot of strain measured at mately LS type are parallel to shallowly dipping band- localities in area 3 shown on Fig. 5. ing. These characteristics are repeated in the core of

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the northern synform on Manitsorssuaq, defining an not accompanied by modification of existing shape augen in Nag. 2 homogenousdeformation, andare fabrics. Large scale folds in area 3 andfurther N believed to be typical of pre-Nagssugtoqidian fabrics probablyformed atthe same time. As is the case throughout the island. further S, the type of shape fabric formed then de- Before Nagssugtoqidian strain, the stretching linea- pends on the intensity of later homogeneousstrain, tion is believed to have had an E-W trendwithin flat-lying and the orientation of the earlier fabrics on which it is foliation and banding in area 3. Evidence for this is superimposed. The non-cylindrical form of early Nag. drawn from the core of the northern synform where 2 folds in area 3 is believed to reflect heterogeneity in Nag. 2 homogeneousdeformation is absent (Fig. 5). early Nag. 2 strain. The orientationof the stretching fabric varies in the N, S and W of the fold core (the E is not exposed), but if Interpretation of the shape fabries the fabrics are returned to a pre-Nagssugtoqidian flat- lying orientation, the stretchingfabric becomes sub- Using these arguments, ellipsoids representing pre- horizontal with an E-W trend. The present orientation Nagssugtoqidian fabrics reorientated by early Nag. 2 of stretching fabrics in the fold core, i.e., sub- folds were deformed in a card shear box. This showed horizontal in the inner parts of northern and southern that existing structures and shapefabrics are consistent limbs and easterly plunging in the inner parts of the with superimposition of Nag. 2 homogeneous strains, western hinge zone, are believed to be the same as increasing from shear strains y = 0.5 to y = 1.5-2.0 those produced by early Nag. 2 folding elsewhere in southwards across area 3, on early Nag. 2 folds trend- area 3. ing ENE with limb dips of about 60"; Formation of early Nag. 2 folds in the shear belt was Fig. 12 shows that S tectonites will form in the

ARCHAEAN STRAIN ELLIPSE AROUND BASIN-LIKE FOLDS AXIAL RATIO, 1.62:1.00:0.62 m banding trace,' '\,,,,R B eaz;z;d //g htnge areas 0 YZ section, XY sectton, $* SOONNW. southern limbs, northern limbs, banding banding vertical L YZ section YZ section horizontal strikes north r------

/ IF.03 I/,! 0.03' %0.38

0.02x 0.08' 0.06 811.7 i' i SO'NNW. L I z X down dtp FIG. 12. Resultant fabrics produced by superimposition of increasingNag. 2 strain (left column) on variably orientated banding-parallel pre-Nagssugtoqidian shape fabrics (top row).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/471/4885865/gsjgs.136.4.0471.pdf by guest on 28 September 2021 486 J. Grocott northern limbs of basin-like folds where YZ sections fabrics in the westernhinge zone of thenorthern of the pre-Nagssugtoqidian fabric are parallel to XZ synform. L > S tectonitesoccur, but the stretching of the superimposed strain, and the banding contain- lineation is never as prominent as in the eastern hinge ing the earlier fabrics dips around 60" to the S. The zone, and nearer the fold core, S > L tectonite fabrics model predicts that up to a superimposed shear strain have formed (Fig. 5). The banding at present dips at shal- of about 1.0, the resultant foliation will lie nearer the low angles, and it is believed that Nag. 2 homogeneous plane of the mineral fabric (which must lie very close strain was superimposed on a section through earlier to the XY plane of the Nag. 2 strain ellipsoid) than fabrics which was close to YZ; i.e., the banding and does banding. Furthermore, as shear strainexceeds XY plane were close to horizontal (Fig. 12). Variation 1.0, the stretchingdirection will change from sub- in shape fabrics is then believed to be due to either horizontal to down the dip of the foliation (Fig. 12). strainmagnitude variation, or to differences in the The pitch of the stretching lineation on the steeply initial orientation of banding. Thus S > L fabrics may dippingfoliation in these limbs variesbetween 8" occur because gneisses near the fold core are further easterly and 49" westerly in the northern fold struc- along the deformation path resulting when Nag. 2 is ture, and between 42" and 75" westerly in the southern superimposed on a YZ fabricsection than L > S fold. Such values are consistent with reorientation of tectonites further W. Alternatively,dips may have an originally shallow stretching fabric by Nag. 2 strain been shallower near the core, prior to homogeneous increasing southwards across area 3 from y= 0.5 to Nag. 2 strain,than further W. Thisexplanation is y > 1.5. Gradual rather than abrupt reorientation of favoured,since Zs at localities 2952 in the W and the stretching fabric is likely to bedue to inexact 2959nearer the fold core, are similar (Fig. 11). coincidence of the YZ and XZ principal planes at L tectonites occur throughout the core of the south- most localities. ern synform since, unlike the northern structure, the In the southern limbs of the basin-like fold struc- fold core is not a Nag. 2 augen. tures, existing S > L shape fabrics are consistent with Gneisses with shape fabric type which approaches superimposition of homogeneous Nag. 2 strain on plane strain occur on the island due to one of three earlier fabrics similar in most respects to those in the circumstances. As describedearlier, samples from northern fold limbs, but having a northerly dipprior to within and close to the core of the northern synform, this strain (Fig. 12). At superimposed shear strains up where Nag. 2 strain is low, exhibit this fabrictype (Fig. to about 1.0 the foliation is predicted to be steeper 11, sample 2893). Plane strain fabrics also occur be- than banding but less steep than themineral fabric. At tween areas of L and S tectoniteswhere pre- shear strains greater than 1.5 the stretching lineation homogeneous Nag. 2 orientation changed with respect becomes down dip (Fig. 12). This assumes an origi- tothe superimposedstrain (Fig. 11, sample 2855). nally sub-horizontal stretching lineation; if this were Finally at high values of superimposed strain the fabric notthe case, a more steeply plunging elongation typeagain approaches plane strain (Fig. 11, sample would result in LS rather than S > L shape fabrics. 2741). The stretching lineation in the eastern hinge zones of the major folds plunged W within the banding after early Nag. 2 folding. Consequently the XZ plane of Conclusion Nag. 2 homogeneous strain is superimposed on a section through the earlier fabric which is between a YZ section with XY horizontal, and a XY section Structural variations, and variations in shape fabrics in with XY vertical and trending N (Fig. 12), depending the Ikert6q shearbelt N of Kingaq, are believed to be on the exact orientation established during early Nag. dueto superimposition of an overthrusting simple 2 folding. At very low values of Nag. 2 strain, L>S shearstrain onto gneisses alreadydeformed by an tectonitesare predicted throughout this range of earlier phase of transcurrent movements. Gradual in- orientations (Fig. 12). At higher strains onlyfabrics creasein the overthrustingdeformation northwards wherethe XY plane was originally steeplydipping allows its progressive effects to be studied in detail. remain in the constrictional field. Within the shear belt, plane strain shape fabrics are It has been argued that shear strain is about 0.5 on the exception rather than the rule, even though the northern Manitsorssuaq and increasessouthwards to component deformations can be shown to be charac- exceed 1.5 in thesouthern basin-like fold. Shape terized by simple shear. Such shape fabric variation is fabrics are firmly in the constrictional field atthe likely to be the case in all similar belts affected by easternends of themajor fold structures, andfor polyphasedeformation. It doesnot mean that such values of superimposed shear strain greater than 0.5, zones cannotbe interpreted with the simple shear Fig. 12 shows that only a relatively steep dip of the model in mind. fabrics prior to homogeneous strain is consistent with N of thenorthern deformationboundary of the the observed resultant fabric. shear belt, relatively low values of overthrusting sim- This argument may be reversed in explaining the ple shearare superimposed on pre-Nagssugtoqidian

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fabrics and structures. Non-plane resultant shape fab- ACKNOWLEDGMENTS.The work in W Greenland was under- rim are produced. One more it is clear thatthe takenas part of the LiverpoolUniversity Precambrian absence of plane strain shape fabrics does not neces- Boundary Programme, supported by NERc grant GR3/1785 sarfiy preclude the of simple shear deforma- and carried outwith the cooperation of the GeologicalSur- tion components within a complex history of vey ofGreenland, during my tenureof a NERCResearch Studentship. I am grateful to my colleagues at Liverpool and deformation. at Swansea for helpful discussion.

References

BAK, J., GROCUIT,J., KORSTGARD, J., NASH,D., SBRENSEN, - & WATTERSON,J. 1974. Stretching fabrics, folds, and K. & WAITERSON,J. 1975. Tectonicimplications of crustal shortening. Tectonophysics, 22, 223-31. Precambrian shear belts in western Greenland. Nature, FLINN,D. 1965. On the symmetry principle andthe deforma- London, 254, 566-9. tion ellipsoid. Geol. Mag. 102, 36-45. BRIDGWATER,D., ESCHER, A. & WATTERSON,J. 1973. Tec- GROC~,J. 1977. The northern boundary of the Ikertdq shear tonic displacements and thermal activity in two contrast- belt, west Greenland. Thesis, Ph.D., Univ.Liverpool ingProterozoic mobile belts fromGreenland. Proc. R. (unpubl.). Soc. London, A 273, 513-33. -in press. Controls of metamorphic grade in shear belts. COWARD,M. P. 1976. Strainwithin ductile shear zones. Rapp.Grplnlands geol. Unders. Tectonophysics, 34, 181-97. RAMSAY, J. G. 1967. Folding and fracturing of rocks. DAVIDSON,L. M. 1978. Granulite facies rocks bordering the McGraw Hill, New York. Ike- shear belt. Thesis, Ph. D., Univ. Liverpool (un- -& GRAHAM,R. H. 1970. Strain variation in shear belts. pub].). Can. J. Earth Sci. 7, 786-813. - & PARK,R. G. 1978. LateNagssugtoqidian stress ROBERTS,B. & SIDDANS,A. W. B. 1971. Fabric structures in orientationderived from deformed granodiorite dykes the LlwydMawr Ignimbrite, Caernarvonshire, North north of Holsteinsborg, W. Greenland. J. geol. Soc. Wales. Tectonophysics, 12, 283-306. London, 135, 283-89. WAITERSON,1968. J. Homogenous deformationof the gneis- DUNNET,D. 1969. A technique of finite strain analysis using ses of Vesterland, S. W. Greenland.Medd. Grmland. elliptical particles. Tectonophysics, 7, 117-36. 175, 72 pp. ESCHER,A., &HER, J. & WATTERSON,J. 1975. The WHKE,S. 1976. The effects of strain on the microstructures, reorientation of the Kanghiut dykeswarm, west fabrics,and deformation mechanisms in quartzites. Greenland. Can. J. Earth Sci. 12, 158-73. Philos. Trans. R. Soc. London, -3, 69-86.

Received 21 June 1978; read 7 February 1979; revisedtypescript received 21 December 1978. JOHNGROCOIT, Department of Geology,University College, Singleton Park, Swansea SA2 8PP.

Discussion

THEPRESIDENT (Professor P. Allen) asked Dr Grocott The possibility that large scale ductile shear zones in if thestructural history in his field area reflected central Greenland represent ductile strains induced in events in thebroader context (e.g. jostling of the response to stresses set up by a collision between the Greenland blocks). Ketilidian block and the rest of Greenland has been DR M. K. WELLS asked the author to comment on explored by Watterson (1978). possible variations of fabric which might be related to In reply to Dr. Wells: Shape fabrics in the region do lithological contrasts of major rock masses in different vary with lithology. However a large proportion of the parts of the region. Was there any evidence to suggest gneisses are granodioritic, and in most of these, as far that the development of some form of layered anisot- asqualitative estimates are concerned, shape fabric ropy in the rocks was a necessary preliminary to the typeand intensity does not vary significantly with development of the second kind of fabric hede- limited change in lithology. To avoid any errors due to scribed? lithological variation in the quantitative study de- In reply to Professor Allen, the AUTHORsaid that: scribed, measurements were restricted to a single rock Formation of ductile shear belts in Greenland is be- type. lieved to be a consequence of early Proterozoic intra- In answering the second part of Dr. Wells' question, plate deformation (Bak et al. 1975). Such intra-plate it is significant that in many areas characterised by deformation may be related to events in southern polyphase deformation, strain axes of later strain often Greenland, where the Ketilidian mobile belt seems to show a degree of coincidence with those of an earlier possess many characteristics of a collision orogeny. deformation. This suggests that development of an

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earlier anisotropy imposes a control on the orientation components respectively, they may be related to a U, of later strains. Evidence described by Mr. D. F. Nash stress trajectory trending N-S and thus imply change (pers. comm.) from the Itivdleq district of the IkertBq in orientation of stress trajectories during progressive shear belt suggests that as strain builds up in a shear deformation. zone local stress tajectory orientation may change. In this area an intense, vertical E-W foliation formed in response to dextral transcurrent shearing during Nag. References 1 deformation. Such movements are presumably re- latedto ahorizontal u1 stresstrajectory orientated BAK, J. K., GROCOTT,J., KORSTGARD,J. A., NASH,D. F., NW-SE. At a later stagemedium and small scale SPIRENSEN,K., & WAITERSON,J. 1975. Tectonicimpli- shear zones formed and reworked the earlier fabrics. cations of Precambrian shear belts in western Greenland. These comprised conjugate vertical sets of shear zones Nature, 254, 566-69. trending NW-SE and NE-SW. Since these shear zones WATTERSON,J. 1978. Proterozoicintraplate deformation in have dextral and sinistral horizontaldisplacement the light of SE Asian neotectonics. Nature, 273,636-40.

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